This invention relates to a liquid crystal display cell, and more particularly to an internal resistance-type heater for a liquid crystal display cell.
It is known that liquid crystal materials exhibit their liquid crystal phase over only a limited temperature range. Below this range these materials are generally solids, and above this range they are ordinary isotropic liquids. The first known liquid crystal materials usually had a liquid crystal phase only within a very limited range above room temperature. Through continued development, many liquid crystal materials are now available that have a liquid crystal phase over considerably wider ranges.
A large variety of materials are now available that exhibit a liquid crystal phase from room temperature to well over 100.degree. C. Therefore, moderately elevated ambient temperatures normally do not present a problem with respect to their use in electro-optic displays. On the other hand, there are only a few materials which exhibit liquid crystal characteristics significantly below room temperature. Of those that do, their viscosities are generally so high at such low temperatures that the response time of an electro-optic display using them is undesirably long. Accordingly, liquid crystal displays have heretofore been limited chiefly to moderate ambient temperature applications, as for example, calculator displays, wristwatch displays, etc.
If a liquid crystal display cell is to be subjected to ambient temperatures colder than room temperature, the cell may have to be heated to be satisfactorily operative. This is particularly true for display cells subjected to the winter temperature extremes of temperate and polar climates. In such climates, exterior visual displays, automobile instrument panel displays, and the like, would require heating means for maintaining the liquid crystal material in its liquid crystal phase and at an appropriate viscosity. For commercial practicality, the heating means must fulfill a number of ancillary requirements to be satisfactory. For example, in automotive applications, it must be more than just effective to protect against moderately low weather temperatures. In such applications, it must be capable of supplying enough heat for temperatures as low as -40.degree. C. Automotive applications require the heating means to be rapidly effective in warming up the liquid crystal cell to its desired operating temperature, even from -40.degree. C. However, it must also be low in manufacturing cost and efficient in operation. Moreover, it should be simple, rugged and reliable, requiring virtually no maintenance in use. It should also be mentioned, of course, that the heating means should not significantly limit the visual display function of the liquid crystal cell. Thus, separate heated enclosures for the liquid crystal cell are generally not desirable.
One technique for providing a reflectance mode electro-optic liquid crystal display device is disclosed in the copending U.S. patent application Ser. No. 681,472, which is entitled "Integrated Thermally Compensated Liquid Crystal Display Device" and which was filed in the names of Dennis F. Dungan and Adolph L. Micheli on Apr. 29, 1976, now U.S. Pat. No. 4,029,393. U.S. Ser. No. 681,472 describes an integrated self-regulating heating element for a liquid crystal cell. The liquid crystal material is sandwiched between a transparent front plane member and a barium titanate back plane member. The barium titanate member forms an integral part of the cell and serves as a source of heat for the cell. Barium titanate has a positive coefficient of electrical resistance that can be tailored for a given application. Electrical current can be passed through the barium titanate member under a constant voltage to provide a self-regulating cell heat source. No separate thermostatic or other type of control is required. Since the voltage applied to the display control electrodes can be a fixed voltage, the voltage source for the display control electrodes can also be used for the barium titanate member too. Thus, it can share the same power supply and even share electrical connections with the electro-optic display electrodes of the cell. This latter invention provides a unique way of heating reflectance mode electro-optic displays.
An integrated resistance-type heater in the liquid crystal cell, such as described in the aforementioned U.S. Ser. No. 681,472 is an attractive and effective heating means. However, we have now discovered a new form of heater integration in a liquid crystal cell, which provides new advantages not available in the prior integrated heater construction. In the new construction, the liquid crystal cell can be used in both the transmissive mode and in the reflectance mode. The heating means in the new integrated means is more intimately associated with the liquid crystal material for more rapid cell warm-up and lower power consumption. It is integrated within the cell itself between facing transparent members enclosing the liquid crystal material of the cell. Moreover, in the new integrated heater structure, two heater elements are included, not just one, for even quicker and more efficient cell warm-up.
The heating elements in our new integrated structure are coatings of resistance material uniquely incorporated in the cell. They are not components that need be specially prepared in advance of cell fabrication, as the barium titanate heating element in the aforementioned U.S. Ser. No. 681,472. Our heating elements are formed during cell fabrication, by techniques similar to those already used in making liquid crystal cells. This contributes to a net cost saving in cell construction.
Proximity to the liquid crystal material for heating purposes can produce electrical fields deleterious to the electro-optic display. However, due to a unique electrical symmetry in our heating elements, such fields are suppressed. Heating voltages in excess of cell threshold voltage can be used without producing fringing electrical fields in the liquid crystal material. Thus, the cell can be strongly heated even while it is concurrently electro-optically displaying an intended message.